1. Field of the Invention
The present invention relates to multicast relay technologies for the Internet, and more particularly to multicast routing which uses PIM-SM.
2. Description of the Related Art
Multicast is concerned with technologies that one packet is transmitted from a transmission terminal and copied at a proper packet relay apparatus at an intermediate relay position, and that the copied packets are output to a plurality of lines to transmit the packets having the same content to a plurality of reception terminals. Since a packet can be transmitted to a plurality of transmission destinations in a smaller amount than that when a packet is transmitted a plurality of times to a plurality of transmission destinations, multicast is suitable for real time multimedia communications requiring a large traffic capacity typically streaming and video conferences.
In order to transmit a multicast packet only to reception terminals originally determined to receive the packet, a packet relay apparatus is required to select proper lines when the packet is output to a plurality of lines. Usually used as a multicast routing protocol for creating a decision criterion of line selection, is a Protocol Independent Multicast Sparse-Mode (PIM-SM) (refer to Non-patent Document 1: IETF, “Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol Specification”, RFC2362).
In PIM-SM, a packet relay apparatus called a Rendezvous Point (RP) plays a major roll in routing. For multicast communications in PIM-SM, each reception terminal to receive a multicast packet transmits a multicast join request to a last-hop router (a packet relay apparatus for accommodating reception terminals). The last-hop router transmits to RP the multicast join request received from the reception terminal. The multicast join request eventually reaches RP via intermediate packet relay apparatus, and each intermediate packet relay apparatus relayed the multicast join request creates a multicast packet transport path in a direction opposite to the relayed direction. In this manner, transport trees of the multicast packet are configured from RP to the last-hop router.
On the side of a multicast transmission terminal, a multicast packet to be transmitted is first sent to a first-hop router (a packet relay apparatus for accommodating transmission terminals). The first-hop router encapsulates once the received multicast packet into a unicast packet (a PIM Register packet), and transmits it to RP. RP extracts the multicast packet from the received PIM Register packet, and outputs the extracted multicast packet to lines toward the last-hop router, in accordance with the configured transport trees.
However, relaying using the above-described RIM Register packet is difficult to yield a high performance. From this reason, PIM-SM operates to directly relay a multicast packet from the first-hop router to PR, without encapsulation, if a flow rate of PIM Register packets exceeds a predetermined value. More specifically, when a communication amount exceeds a threshold value, RP transmits a PIM Join-Prune packet to the first-hop router to configure multicast packet transport trees from the first-hop router to RP. In addition, RP sends a command (PIM Register-Stop packet) to the first-hop router to stop encapsulation at the first-hop router and realize direct relay of a multicast packet in accordance with the configured transport trees.
If a line between the first-hop router and RP and the first-hop router and RP themselves are unstable, a PIM Register-Stop packet from RP does not reach the first-hop router in some case. In this case, a PIM Register packet transmission process at the first-hop router will not stop, and a CPU load on the first-hop router becomes high. There arises therefore a problem that the initially unstable first-hop router and RP become more unstable because of the CPU load caused by PIM Register packet transmission.
PIM-SM specifications (Non-patent Document 1) solve this problem by using a BootStrap Router (BSR). BSR is a packet relay apparatus which periodically advertises RP information in a network. In accordance with the information supplied from BSR, the first-hop router selects a destination RP of a PIM Register packet. In the unstable network state described above, the RP information from BSR will not reach the first-hop router, and will time out eventually. It is therefore possible to suppress PIM Register packet transmission from the first-hop router to RP.
This problem can also be solved by using Protocol Independent Multicast Source-Specific Multicast (PIM-SSM) (Non-patent Document 2: IETF “An Overview of Source-Specific Multicast (SSM)”, RFC3569) which extends the PIM-SM specifications and realizes multicast routing without involvement of RP. On the assumption that a multicast join request from each multicast reception terminal contains an IP address of a multicast transmission terminal in PIM-SSM, the last-hop router transmits the multicast join request directly to the first-hop router to thereby dispense with PIM Register transmission from the first-hop router to RP.
This problem is also solved by controlling a PIM Register transmission rate at the first-hop router, in Cisco Systems (Non-patent Document 3: Cisco Systems, “Cisco IOS IP Multicast Command Reference Release 12.4T”, pp. 179-180). By regulating a PIM Register transmission upper limit at the first-hop router, it is possible to prevent an overload of the first-hop router to be caused by a PIM Register packet transmission load.